The invention relates to a tire having a carcass reinforcement, preferably a radial one, intended to be fitted more particularly on a passenger vehicle.
In addition to the carcass reinforcement, which is anchored within each bead to at least one bead wire, said tire comprises radially to the outside of said carcass reinforcement a crown reinforcement composed of at least two so-called working crown plies formed of inextensible reinforcement elements, which are parallel to each other within each ply and are crossed from one ply to the next, forming angles which may be between 10xc2x0 and 45xc2x0 with the circumferential direction. The tread, which is arranged radially to the outside of said crown reinforcement, is joined to the two beads of the tire by means of two sidewalls on either side of the equatorial plane.
As is widely known and used, the junction with the shoulders of the tire, between the rubber mix of the tread and the two rubber sidewall mixes, can be brought about in two ways. A first method consists in folding the radially upper edges of the sidewall mixes over the axially outer edges of the tread mix. Said method, when laying the sidewall mixes on the cylindrical shaping drum for the non-vulcanized carcass blank, requires the non-adhesion of the upper edges of the sidewall mixes, then the folding of said non-adhering edges on to the wings of tread mix during the finishing of the non-vulcanized tire blank. The operations referred to above may in some cases be tricky.
A second method frequently used consists firstly of folding down the wings of tread mix over the upper edges of the sidewall mixes, said edges, during the building of the tire blank, being glued to the radially subjacent mixes. The junction between the tread mix and the sidewall mixes, which is on the outer wall of the sidewall in the form of a more or less regular circle, is then covered with a layer of rubber mix or joint cover, of a quality close to that of the sidewall mixes. This latter solution permits good strength of the junction and good life of the tire at this level, but requires the laying of an additional product, and therefore a certain disadvantage in terms of material and production costs.
Despite the possible tapering of the tread wings so as to facilitate the folding over thereof, since the materials used for the tread mixes and sidewall mixes are generally very different and the mechanical stresses due to the loading of the tire in the contact area and imposed on the shoulders of said tire are high, it has not been possible to dispense with the use of the joint cover over the junction between the tread and sidewalls, since the life of the junction is not judged sufficient for intensive, severe highway use.
The object of the invention is to dispense with the use of the joint cover while using the second method of building the tire blank.
The tire according to the invention, comprising a carcass reinforcement anchored within each bead to a bead wire, a crown reinforcement and a tread joined to two beads by means of two sidewalls, the axially outer edges of the single mix of the tread being folded over on to the radially outer edges of the rubber mixes of the sidewalls, is characterized in that the circular junction between the sole rubber tread mix and the rubber sidewall mix is positioned on the axially outer wall of the tire such that its radius RC lies firstly between 0.9 RS+0.1 RB and 0.8 RS+0.2 RB and secondly between RSS and 0.9 RSS+0.1 RB, RS being the equatorial crown radius of the tread, RSS the equatorial radius of the center line of the carcass reinforcement and RB the radius of the bead seat measured on the line perpendicular to the axis of rotation of the tire passing through the center of gravity of the cross-section of the bead wire.
Such positioning permits a serious reduction in the deformation which the circular junction between the tread mix and the sidewall mix undergoes, and consequently substantially improved strength of the bond.
Preferably, the circular junction between the mixes in question is close to at least one circumferential groove or channel, the mean radius RR of which is between RC+10 mm and RCxe2x88x9210 mm, and the depth of which is between 10 and 30% of the total sidewall thickness at the radius RR. It is advantageous for the channel to have a mean radius of between RC and RCxe2x88x9210 mm in order to have maximum effectiveness. The cross-section of said groove may be semicircular, or alternatively may, when viewed in meridian section, have a shape which is a succession of arcs of a circle.
The characteristics of the invention will be better understood with reference to the following description, which refers to the drawings which illustrate in non-limitative manner examples of embodiments of the invention.